building documentation locally is broken

[snoek09]

The build files have been moved to _build.
Running make html from _build now causes a Sphinx error:

Sphinx error:
master file /home/user/git/symfony-docs/_build/index.rst not found
Makefile:45: recipe for target 'html' failed
make: *** [html] Error 1

[snoek09]

Using commands from travis-ci build:

pip install sphinx~=1.3.0 git+https://github.com/fabpot/sphinx-php.git
sphinx-build -nW -c _build/ -b html -d _build/doctrees . _build/html

[xabbuh]

see also #6799